Circuit for use in frequency or phase detector

ABSTRACT

A tracking data cell ( 10 ) comprising:—a pair of track and hold circuits ( 1, 1 ′) coupled to a first multiplexer ( 5 ),—a clock signal (H+, H−) being inputted substantially in anti-phase in the respective track and hold circuits ( 1, 1 ′) for determining a receipt of a data signal (D+, D−) having a rate, —said track and hold circuits ( 1, 1 ′) providing an output signal (O) having a substantially half rate.

The invention relates to a tracking data cell and to a phase locked loop circuit comprising such a cell. The invention further relates to a track and hold circuit included in such a phase locked loop.

Phase locked loops (PLL) are widely used in modern technology. It typically comprises a loop connection of at least a voltage control oscillator (VCO), a phase detector and a low-pass filter. Additionally, PLL may comprise a so-called frequency loop including the VCO coupled to a frequency detector and to another low-pass filter. VCO may generate signals having different shapes as sinusoids, rectangle, triangle.

U.S. Pat. No. 5,006,819 discloses a PLL comprising as VCO a ramp generating circuitry for generating a dual slope ramp signal having alternating positive and negative slopes i.e. triangle shaped signals. The level of the control signal controls the slopes. The PLL further comprises sampling circuitry responsive to sample command pulses for providing a sample output representative of the level of the dual ramp signal at the time of sampling. A main drawback of the PLL disclosed in U.S. Pat. No. 5,006,819 is that triangle shaped signals are very hard to be obtained at relatively high frequencies e.g. GHz range. Hence, there is a need to obtain a PLL circuit that works in a relatively high frequency range maintaining the circuit complexity at a relatively low level i.e. being relatively easy to be implemented at a reasonable price.

This object is obtained using a tracking data cell (10) comprising:

-   -   a pair of track and hold circuits coupled to a first         multiplexer,     -   a clock signal being inputted substantially in anti-phase in the         respective track and hold circuits for determining a receipt of         a data signal having a rate,     -   said track and hold circuits providing an output signal having a         substantially half rate.

The clock signal shape is not restricted to triangle form and therefore the circuit may be used with substantially any type of signal shapes. Data and clock recovery (DCR) circuits may be considered as PLL for Non Return to Zero (NRZ) signals, which are used in modern optical communication networks at frequencies situated in GHz range. In this frequency range it is relatively hard to implement DCR circuits using the same hardware as in lower frequency ranges as e.g. flip-flops. Any flip-flop has some fundamental delays as a delay determined by a clock signal and a decision delay i.e. a time necessary for an output to become stable. These delays are technology dependent and therefore they cannot be made very small. Hence, there is a need to find devices for replacing flip-flops in relatively high frequencies devices as DCRs. In high-speed decision circuits i.e. flip-flops, latches, which work at full-rate of an input signal the most difficult function is memorizing. The decision circuits have to decide at full speed and to track the input data signal sufficiently fast such that the setup and hold conditions of the decision circuits e.g. latches, flip-flops are not violated. Hence, in these conditions, a track and hold circuit helps because it does not to take a decision whether the input data is at a low or at a high level.

In an embodiment, the tracking data cell is used in a PLL comprising a first tracking data cell and a second tracking data cell receiving an input signal and being controlled by a respective quadrature clock signals generated by a voltage controlled oscillator. The first tracking data cell is coupled to a hard limitter providing a binary data output signal. The second tracking data cell is coupled to a to delay element providing an input signal for a pair of track and hold circuits. The track and hold circuits are controlled by the binary output signal and provide a frequency correction signal for the voltage-controlled oscillator via a low-pass filter. The above-mentioned embodiment is useful in optical communication networks in said data and clock recovery circuits, which are phase locked loops for non return to zero signals. Because the frequencies involved here may be e.g. according to IEEE 802.16 as 10 to 66 GHz the half rate concept is very useful both for emitters and receivers.

In another embodiment of the invention, the PLL further comprises a frequency error detector including a first track and hold circuit and a second track and hold circuit receiving the frequency correction signal and being controlled by the binary output signal, said first and second track and hold circuits being coupled to a multiplexer means controlled by the binary output signal, the multiplexer means being coupled to a slicer providing a signal which is subtracted from the frequency correction signal in a subtractor, the subtractor providing a signal indicative for a frequency error between the frequency correction signal and the binary output signal. The phase detector output is memorized and subtracted after the first track and hold and multiplexer combination, which has the task to measure the gradient of the phase detector output and to generate a positive or a negative error when a cycle slip occurs in the phase detector output. In lock, the output of the frequency detector toggles between a negative and a positive value generating a zero average signal after a low pass filter. This is the reason to consider a possible gating mechanism to detect frequency lock condition and to cut-off the frequency loop if the frequency error is smaller than a threshold value.

In an embodiment of the invention the phase detector is implemented using Track and hold circuits. The phase detector comprises a first input circuit and second input circuit The first and second input circuits receive respective quadrature clock signals and are controlled by input data signal. Said first and second input circuits provide respective first output signal and second output signal. The first output signal and its inverse replica are inputted to an output multiplexer controlled by the second output signal via a hard limitter. The output multiplexer provides a signal indicative for a phase error between the input data signal and clock signal.

According to the value of the second signal the phase detector output will be the first signal or the inverted replica of the first signal. When the second signal has a positive value i.e. a projection on the x-axis e.g. +Δ then the output of the phase detector is a equal to the value of the first signal. When the second signal has a negative value e.g. −Δ then it is necessary to invert the value of the first signal at the output of the phase detector. Hence, one gets an output voltage that is linearly proportional with the phase error.

Preferable, the track and hold circuit comprises a linear amplifier receiving a differential analog signal and being controlled by a first binary clock signal having a phase, signal to a pseudo latch circuit in a first phase of the first binary clock signal, said pseudo latch circuit being controlled by a second binary clock signal for memorizing the input signal and providing a differential output signal substantially equal with the input signal during a second phase of the first binary clock signal, the second binary clock signal being substantially in anti-phase with the first binary clock signal i.e. phase shifted by 90 degrees. The T/H circuit uses both phases of a clock signal and therefore its operating frequency substantially doubles in comparison with the T/H circuit that use single ended architectures. Furthermore, there is no feedback and therefore the T/H circuit is not slowed down by the feedback. Instead the signal provided by the linear amplifier is directly feed-forward to the pseudo latch circuit.

In an embodiment of the invention the linear amplifier comprises a first common source pair of transistors biased in their common source terminals by a switchable current source via a first switch controlled by the first binary clock signal and receiving at their gates the differential analog signal, the linear amplifier further comprising a common drain transistor having a gate coupled via substantially equal resistors to the respective gates of the pair of transistors for determining drain currents flowing through the pair of transistors. Let us consider that the pair of transistors comprises transistors having the same area and that the common drain transistor has a different area. Let us further note the common resistors as R and the current flowing through it as i. The following relations may be written:

$\begin{matrix} \left\{ \begin{matrix} {V_{iD} = {2{iR}}} \\ {V_{iD} = {V_{T} + \sqrt{\frac{2i_{1}}{\beta_{1}}} - V_{T} - \sqrt{\frac{2i_{1}}{\beta_{3}}} + {iR}}} \\ {V_{iD} = {{iR} + V_{T} + \sqrt{\frac{2i_{3}}{\beta_{3}}} - V_{T} - \sqrt{\frac{2i_{2}}{\beta_{1}}}}} \\ {i_{3} = {I_{B} - \left( {i_{1} + i_{2}} \right)}} \end{matrix} \right. & (1) \end{matrix}$ In relations (1) β₁, and β₃ are coefficients related to the dimensions of the pair of transistor and common drain transistor, respectively. V_(T) is a threshold voltage of the transistors. i₁ and i₂ are the currents through the pair of transistors. V_(iD) is the differential input signal and I_(B) is the current provided by the switcheable current source and i₃ is a current through the common drain transistor. i₃ is quadratic with respect to the input differential voltage V_(iD) as in eq. (2).

$\begin{matrix} {i_{3} = {\frac{I_{B}}{1 + {2\left( \frac{w_{1}}{w_{3}} \right)}}\left( {1 - \frac{\beta_{1}v_{ID}^{2}}{4\; I_{B}}} \right)}} & (2) \end{matrix}$

The differential output current depends on the differential input voltage as shown in equation (3):

$\begin{matrix} {i_{OD} = {{i_{1} - i_{2}} = {\beta_{1}{\sqrt{\frac{2I_{B}}{\beta_{3}\left( {1 + {2\frac{w_{1}}{w_{3}}}} \right)}} \cdot v_{ID}}\sqrt{1 - \frac{\beta_{1}v_{ID}^{2}}{4I_{B}}}}}} & (3) \end{matrix}$ It is to be mentioned here that the term β₁ V_(iD) ² has dimension of a current. It is further observed that if I_(B) is chosen such that β₁ V_(iD) ²<<I_(B) then relation (3) reduces to relation (4).

$\begin{matrix} {i_{OD} \approx {\beta_{1}{\sqrt{\frac{2I_{B}}{\beta_{3}\left( {1 + {2\frac{w_{1}}{w_{3}}}} \right)}} \cdot v_{ID}}}} & (4) \end{matrix}$

Hence, the current is linearly dependent on the differential input voltage.

In another embodiment of the invention the pseudo latch circuit comprises a second common source pair of transistors being biased in their common source terminals by a switcheable current source via a second switch controlled by the second binary clock signal and receiving at their gates the signal provided by the linear amplifier the pseudo latch circuit further comprising a common drain transistor having a gate coupled via substantially equal resistors to the respective gates of the second differential transistor pair for reducing bias currents through the second transistor pair, the second transistor pair being cross coupled. Cross coupling transistor pair i.e. a drain of a transistor is coupled to a gate of the other transistor and reciprocally determines a positive feedback in the pseudo latch circuit. Similarly as shown in previous relations 1-4, a current through the common drain transistor determined the currents through the second transistor pair. The current is chosen such that the overall amplification of the stage is substantially 1. Hence, the pseudo latch circuit does not commute as a standard latch circuit in which a drain of the transistor pair has a high voltage and the other drain has a low voltage and reciprocally and when the current through the transistor pair has substantially larger values. Hence, the pseudo latch circuit provides a signal substantially equal with the input signal.

In another embodiment of the invention the linear amplifier further comprises a pair of capacitors cross-coupled between a drain of a transistor of the first transistor pair and the gates of the other transistor of the first transistor pair, respectively for reducing crosstalk currents at the amplifier's output. In this case, two extra dummy transistors to generate a parasitic capacitance equal to the drain-gate capacitance of the tracking transistors, were added. The capacitance takes the injected charge at the drains of the first pair of transistors such that the net to crosstalk current flowing at the output is zero.

In an embodiment of the invention, the track and hold circuit comprises a cascaded coupling of two substantially identical linear amplifiers for better isolating the input data from the pseudo latch circuit. The function of the added linear stage is to better isolate the input signal from the output signal during the hold period. The gain of the added stage is chosen substantially equal to one and therefore, in the tracking mode, the output of the combination of the two linear stages tracks the input signal. The total parasitic capacitance between input and output is reduced due to the cascade connection of the two stages and the neutrodination effect of the added capacitors. In a practical implementation an extra capacitor of 500 fF may be added at the differential output to improve the accuracy during the hold mode.

The above and other features and advantages of the invention will be apparent from the following description of the exemplary embodiments of the invention with reference to the accompanying drawings, in which:

FIG. 1 depicts a tracking data cell, according to the invention,

FIG. 2 depicts waveforms of a data transition tracking loop, according to the invention,

FIG. 3 depicts output waveform of a phase detector, according to the invention,

FIG. 4 depicts a phase locked loop, according to the invention,

FIG. 5 depicts output signals of a phase detector used in a phase locked loop, according to the invention,

FIG. 6 depicts a frequency error detector, according to the invention,

FIG. 7 depicts a phase error detector according to the invention, and

FIG. 8 depicts quadrature vectors generated by a phase detector, according to the invention.

FIG. 9 depicts a block diagram of a track and hold circuit according to the invention,

FIG. 10 depicts a transistor level of a linear amplifier, according to the invention,

FIG. 11 depicts a transistor level of a first embodiment of a track and hold.

circuit, according to the invention,

FIG. 12 depicts a second embodiment of the track and hold circuit, according to the invention, and

FIG. 13 depicts a third embodiment of track and hold circuit, according to the invention.

FIG. 1 depicts a tracking data cell 10, according to the invention. The tracking data cell 10 comprises a pair of track and hold circuits 1, 1′ coupled to a first multiplexer 5. A clock signal H+, H− is inputted substantially in anti-phase in the respective track and hold circuits 1, 1′ for determining receipt of a data signal D+, D− having a rate. The track and hold circuits 1, 1′ provide an output signal O having a substantially half rate. In high-speed decision circuits, which work at full-rate of an input signal as e.g. flip-flops, latches the most difficult function is memorizing. The decision circuits have to decide at full speed and to track the input data signal sufficiently fast such that the setup and hold conditions of the decision circuits e.g. latches, flip-flops are not violated. Hence, in these conditions, a track and hold circuit helps, it holds the input data and does not to take a decision whether the input data is at a low or at a high level. Here the output signal O of the multiplexer 5 is substantially equal with the input signal D+, D−. This circuit has the advantage that the outputs of the track and hold circuits 1, 1′ may be used to generate a half-rate version of the input signal by adding the two track and hold circuits 1, 1′ clocked in anti-phase by the clock signals H+, H−.

FIG. 4 depicts a phase locked loop (PLL) 100, according to the invention. Let us suppose that there are quadrature input signals D+, D−. It was further supposed that the system is provided at its input with a limiter/buffer that are not figurated for band-limiting the input signals and as a matter of consequence, the shape of the input data is substantially sinusoidal as shown in FIG. 2. According to FIG. 2 the quadrature sample is negative if the clock is early, the quadrature sample is zero when the clock is on-time and the quadrature sample is positive when the clock is late. This situation corresponds to a positive transition of the input signal. One can generate a phase error following the next rules:

-   -   If the input signal makes no transition then hold the previous         phase-error value,     -   Pass the quadrature sample if the input data signal Makes a low         to high transition, and     -   If the input data makes a high to low transition pass the         negative of the quadrature sample to a phase-detector output.

The output of the phase detector has a monotonic characteristic over a bit period as it is shown in FIG. 3. Based on the above rules for generating the phase error a PLL 100 may be derived, as shown in FIG. 4. The PLL comprises a first tracking data cell 10 and a second tracking data cell 10′. The tracking data cells 10, 10′ receive an input signal D+, D− and are controlled by respective quadrature clock signals Hi, Hq generated by a voltage-controlled oscillator (VCO). The first tracking data cell 10 is coupled to a hard limitter 11 providing a binary data out signal DO. The second tracking data cell 10′ is coupled to a to delay element 12 providing an input signal for a pair of track and hold circuits 1, 1′. The track and hold circuits 1, 1′ are controlled by the binary output signal DO and provide a frequency correction signal E for the voltage controlled oscillator VCO via a low-pass filter LPF. The pair of track and hold circuits 1, 1′ are coupled to a multiplexer 5 to implement a phase detector as was previously described. In FIG. 5 the output signal E of the phase detector used in the PLL, is shown. The above-mentioned embodiment is useful in optical communication networks in said data and clock recovery circuits, which are phase locked loops for non return to zero signals. Because the frequencies involved here may be according to IEEE 802.16 as 10 to 66 GHz and the half rate concept is very useful both for emitters and receivers.

FIG. 6 depicts a frequency error detector 50, according to the invention. The frequency error detector 50 includes a first track and hold circuit 30 and a second track and hold circuit 30′ receiving the frequency correction signal E. The track and hold circuits 30, 30′ are controlled by the binary output signal DO. The first and second track and hold circuits 30, 30′ are coupled to a multiplexer 25 controlled by the binary output signal DO. The multiplexer 25 being coupled to a slicer 35 providing a signal which is subtracted from the frequency correction signal E in a subtractor S. The subtractor S provides a signal FD indicative for a frequency error between the frequency correction signal E and the binary output signal DO. The phase detector output E is memorized and subtracted after the first track and hold and multiplexing process, which has the task to measure the gradient of the phase detector output E and to generate a positive or a negative error when a cycle slip occurs in the phase detector output signal. In lock, the output of the frequency detector toggles between a negative and a positive value generating a zero average signal after a low pass filter LPF. This is the reason to consider a possible gating mechanism to detect frequency lock condition and to cut-off the frequency loop if the frequency error is smaller than a threshold value.

FIG. 7 depicts a phase error detector according to the invention. The phase detector comprises a first input circuit 500 and second input circuit 500′. First and second input circuits 500, 500′ receive respective quadrature clock signals Hq, Hi. The first and second circuits 500, 500′ are controlled by input data signal D and provide respective first output signal A and second output signal B. The first output signal A and its inverse replica i.e. obtained via an inverter 60 is inputted to an output multiplexer OM controlled by the second output signal B via a hard limitter 250. The output multiplexer OM provide a signal PD indicative for a phase error between the input data signal Hq, Hi and clock signal D. The selection signal of the output multiplexer is a quantized version of the B signal. Therefore, we can use a limiter or a digital multiplexer to generate the signal B. FIG. 8 presents the vector diagram for three possible situations: late, in-phase and early. According to the value of second signal B the phase detector output is either A or the inverted value of A. When second signal B has a positive value i.e. the projection on the x-axis e.g. B=Δ then the output of the phase detector is a equal to the value of A. When the second signal B has a negative value e.g. B=−Δ then it is necessary that the first signal to be inverted. Hence, we get an output signal linearly proportional to the phase error. The logic behind the generation of the phase error is presented in Table 1.

TABLE 1 B = +Δ A = −Δ EARLY PD = −Δ B = +Δ A = +Δ LATE PD = +Δ B = −Δ A = −Δ LATE PD = +Δ B = −Δ A = +Δ EARLY PD = −Δ

In FIG. 8 it is presented a vector diagram for B=Δ and B=−Δ.

FIG. 9 depicts a block diagram of a preferred track and hold circuit 1 according to the invention. The track and hold circuit 1 comprises a linear amplifier 2, which receives a differential analog signal D+, D−. The linear amplifier 2 is controlled by a first binary clock signal H+having a first phase. The linear amplifier 2 provides a feed-forward input signal substantially equal with the differential analog signal D+, D− to a pseudo latch circuit 3 in the first phase of the first binary clock signal H+. The pseudo latch circuit 3 is controlled by a second binary clock signal H− for memorizing the input signal. The pseudo latch circuit 3 provides a differential output signal LD+, LD− substantially equal with the input signal D+, D− during a second phase of the first binary clock signal H−. The second binary clock signal is substantially in anti-phase with the first binary clock signal H+i.e. phase shifted with 90 degrees. The T/H circuit uses both phases of a clock signal and therefore its operating frequency substantially doubles in comparison with the T/H circuit in U.S. Pat. No. 6,489,814. Furthermore, there is no feedback and therefore the T/H circuit is not slowed down by the feedback. Instead the signal provided by the linear amplifier is directly feed-forward to the pseudo latch circuit 2.

FIG. 10 depicts a transistor level of a preferred linear amplifier 2, according to the invention. The linear amplifier 2 comprises a first common source pair of transistors T1, T2 biased in their common source terminals by a switcheable current source I_(DC) via a first switch S1 controlled by the first binary clock signal H+. The first common source pair of transistors T1, T2 receives at the gates of the transistors the differential input signal D+, D−. The linear amplifier 2 further comprises a common drain transistor T3 having a gate coupled via substantially equal resistors R to the differential input signal D+, D− for determining drain currents flowing through the pair of transistors. As it was shown in relations 1 to 4, the output current is linearly dependent on the differential voltage at the input. Resistors R_(L) converts the output current into a voltage that is further provided to the pseudo latch circuit 3. The pseudo latch circuit 3 comprises, as shown in FIG. 11, a second common source pair of transistors T4, T5 being biased in their common source terminals by a switcheable current source I_(DC) via a second switch S2 controlled by the second binary clock signal H−. The second common source pair of transistors T4, T5 receives at the gates of the transistors the signal provided by the linear amplifier 2 e.g. from the drains of the first pair of common source transistors T1, T2. The pseudo latch circuit 3 further comprises a common drain transistor T6 having a gate coupled via substantially equal resistors Rg to the respective gates of the second common source pair of transistors T4, T5 for reducing bias currents through the transistors pair T4, T5. The second transistor pair T4, T5 is cross-coupled i.e. a drain of a transistors e.g. is coupled to a gate of the other transistor e.g. T5 and reciprocally. As it was shown in previous relations 1-4, a current through the common drain transistors T6 determines the currents through the second transistor pair T4, T5. The current is chosen such that the overall amplification of the stage is substantially 1. Hence, the pseudo latch circuit does not commute as a standard latch circuit in which one drain of the transistor pair has a high voltage and the other drain has a low voltage and reciprocally and when the current through the transistor pair has substantially larger values. Hence, the pseudo latch circuit 3 provides a signal OUTP, OUTN substantially equal with the input signal D+, D−. The capacitor CAP represents the input capacitance of a stage following the track and hold circuit. When relative lower frequencies are involved, an additional capacity CAP may be added for improving memorizing process in a hold state.

FIG. 12 depicts a second embodiment of the track and hold circuit 1, according to the invention. The linear amplifier 2 further comprises a pair of capacitors cross-coupled between a drain of a transistor of the first transistor pair T1, T2 and the gates of the other transistor of the first transistor pair T2, T1, respectively for reducing crosstalk currents at the amplifier's output. In this case, two extra dummy transistors C to generate a parasitic capacitance equal to the drain-gate capacitance of the tracking transistors T1, T2, were added. The capacitance takes the injected charge at the drains of the first pair of transistors T1, T2 such that the net to crosstalk current flowing at the output e.g. drains of T1, T2 is substantially zero.

FIG. 13 depicts a third embodiment of track and hold circuit 1, according to the invention. The track and hold circuit 1 comprising a cascaded coupling of two substantially identical linear amplifiers 2, 2′, for better isolating the input signal D+, D− from the pseudo latch circuit 3. The gain of the added stage is chosen equal to one and therefore, in the tracking mode, the output of the combination of the two linear stages tracks the input signal. The total parasitic capacitance between input and output is reduced due to the cascade connection of the two stages 2, 2′ and the neutrodination effect of the added capacitors C. In a practical implementation an extra capacitor CAP of e.g. 500 fF may be added at the differential output to improve the accuracy during the hold mode.

It is remarked that the scope of protection of the invention is not restricted to the embodiments described herein. Neither is the scope of protection of the invention restricted by the reference numerals in the claims. The word ‘comprising’ does not exclude other parts than those mentioned in the claims. The word ‘a(n)’ preceding an element does not exclude a plurality of those elements. Means forming part of the invention may both be implemented in the form of dedicated hardware or in the form of a programmed purpose processor. The invention resides in each new feature or combination of features. Aspects shown but not claimed may be claimed in a copending application. 

1. A tracking data cell (10) comprising: a pair of track and hold circuits (1, 1′) coupled to a first multiplexer (5), wherein each of the track and hold circuits (1, 1′) comprises: a linear amplifier (2) to receive differential analog signal (D+, D−) under control by a first binary clock signal (H+) having a first phase, the linear amplifier (2) to provide a feed-forward input signal substantial equal with the differential analog signal (D+, D−) to a pseudo latch circuit (3) in the first of the first binary clock signal (H+), the pseudo latch circuit (3) under control by a second binary clock signal (H−), the pseudo latch circuit (3) to memorize the input signal and to provide a differential output signal (LD+, LD−) substantially equal with the feed-forward input signal in a second phase of the first binary clock signal (H−), the second binary clock signal substantially in anti-phase with the first binary clock signal (H+), a clock signal (H+, H−) being inputted substantially in anti-phase in the respective track and hold circuits (1, 1′) for determining a receipt of a data signal (D+, D−) having a rate, said track and hold circuits (1, 1′) providing an output signal (O) having a substantially half rate relative to the rate of the data signal (D+, D−).
 2. A tracking data cell as claimed in claim 1, wherein the linear amplifier (2) comprises a first common source pair of transistors (T1, T2) biased in their common source terminals by a switchable current source (I_(DC)) via a first switch (S1) controlled by the first binary clock signal (H+) and receiving at their gates the differential analog signal (D+, D−), the linear amplifier (2) further comprising a common drain transistor (T3) having a gate coupled via substantially equal resistors (R) to the gates of the first common source pair of transistors (T1, T2) for determining drain currents flowing through the pair of transistors (T1, T2).
 3. A tracking data cell as claimed in claim 1, wherein the pseudo latch circuit (3) comprises a second common source pair of transistors (T4, T5) being biased in their common source terminals by a switchable current source (I_(DC)) via a second switch (S2) controlled by the second binary clock signal (H−) and receiving at their gates the signal provided by the linear amplifier (2), the pseudo latch circuit (3) further comprising a common drain transistor (T6) having a gate coupled via substantially equal resistors (Rg) to the respective gates of the second differential transistor pair (T4, T5) for reducing bias currents through the second transistor pair (T4, T5), the second transistor pair (T4, T5) being cross coupled.
 4. A tracking data cell as claimed in claim 2, wherein the linear amplifier (2) further comprises a pair of capacitors cross-coupled between a drain of a transistor of the first transistor pair (T1, T2) and the gates of the other transistor of the first transistor pair (T2, T1), respectively for reducing crosstalk currents at the amplifier's output.
 5. A tracking data cell as claimed in claim 4 comprising a cascaded coupling of two substantially identical linear amplifiers (2).
 6. A Phase Locked Loop comprising a first tracking data cell (10) and a second tracking data cell (10′) as claimed in claim 1 receiving an input signal (D+, D−) and being controlled by a respective quadrature clock signals (Hi, Hq) generated by a voltage controlled oscillator (VCO), the first tracking data cell (10) being coupled to a hard limiter (11) providing a binary data out signal (DO), the second tracking data cell being coupled to a to delay element (12) providing an input signal for a pair of track and hold circuits (1, 1′), the track and hold circuits being controlled by the binary output signal (DO) and providing a frequency correction signal (E) for the voltage controlled oscillator (VCO) via a low-pass filter (LPF).
 7. A Phase Locked Loop comprising a first tracking data cell (10) and a second tracking data cell (10′) as claimed in claim 1 receiving an input signal (D+, D−) and being controlled by a respective quadrature clock signals (Hi, Hq) generated by a voltage controlled oscillator (VCO), the first tracking data cell (10) being coupled to a hard limiter (11) providing a binary data out signal (DO), the second tracking data cell being coupled to a to delay element (12) providing an input signal for a pair of track and hold circuits (1, 1′), the track and hold circuits being controlled by the binary output signal (DO) and providing a frequency correction signal (L) for the voltage controlled oscillator (VCO) via a low-pass filter (LPF) further comprising a frequency error detector comprising an input circuit (50) including a first track and hold circuit (30) and a second track and hold circuit (30′) as claimed in claim 1, receiving the frequency correction signal (E) and being controlled by the binary output signal (DO), said first and second track and hold circuits (30, 30′) being coupled to a multiplexer means (25) controlled by the binary output signal (DO), the multiplexer means (25) being coupled to a slicer (35) providing a signal which is subtracted from the frequency correction signal (E) in a subtractor (S), the subtractor (S) providing a signal (FD) indicative for a frequency error between the frequency correction signal (E) and the binary output signal (DO).
 8. A phase detector comprising a first input circuit (500) and second input circuit (500′) as claimed in claim 7, first and second input circuits (500, 500′) receiving respective quadrature clock signals (Hq, Hi), being controlled by input data signal (D) and providing respective first output signal (A) and second output signal (B), the first output signal (A) and its inverse replica being inputted to an output multiplexer (GM) controlled by the second output signal (B) via a hard limiter (250) and providing a signal (PD) indicative for a phase error between the input data signal and clock signal. 